1. Field of the Invention
This present invention relates to a technique for equalizing waveform of signals read from recording medium such as optical discs, etc.
2. Description on Related Art
In a information reproducing apparatus for reproducing information recorded on recording medium such as optical discs, a slice method has been adopted, in which a signal waveform level greater than a specified value is judged as “1” and that smaller than a specified value is judged as “0”. However, in this method, it is difficult to reproduce data at a high reliability from the recording medium with remarkably improved recording density. Therefore, in recent years, attention is attracted to Partial Response Maximum Likelihood (hereinafter referred to as PRML) method that can reproduce the data at a high reliability. PRML method is a technique utilized for a signal processing technique of recording medium with increased density, which is utilized in, for example, a digital recording camera integrated type VTR, rewritable optical discs, not to mention a hard disk drive (hereinafter referred to as HDD). As the recording density increases, a need for reproducing data correctly from reproduction signals with low signal-to-noise (SIN) ratio or nonlinear reproduction signals is increased.
FIG. 16 is a block diagram of a general configuration of an information reproducing apparatus 181 using PRML method. First of all, an optical pickup 183 irradiates optical disc 182 with laser beam. Information reproducing apparatus 181 detects intensity of the reflected light, reads the information (data) recorded on optical disc 182, converts into electrical signals, and outputs to Front End Processor (hereinafter referred to as FEP) 184. FEP184 amplifies the electrical signals read and adjusts its gain. FEP184 further processes to remove noise components of unrequired high frequency band and to emphasize the required signal frequency band. The output signals from FEP184 are converted to digital signals by analog/digital (A/D) converter 185 and entered into waveform equalizer 186. Waveform equalizer 186 waveform-equalizes waveforms of the digital signals to the preset PR characteristics. Maximum likelihood decoder 187 decodes waveform-equalized signals to PR characteristics and outputs as the reproduction data.
Waveform equalizer 186 of information reproducing apparatus 181 generates waveform in such a manner as to achieve the desired PR characteristics, that is, PR (3, 4, 4, 3) characteristics. FIG. 17 is a block diagram of an exemplary configuration of waveform equalizer 186. Waveform equalizer 186 is called as a transversal filter or Finite Impulse Response(FIR) filter. Waveform equalizer 186 generally includes a plurality of delay elements 192, a plurality of equalizing coefficients (coefficients A through E) for achieving the desired PR characteristics, a plurality of multipliers 193 for multiplying equalizing coefficients by the output of delay element 192, and an adder 194 for adding outputs of a plurality of multipliers 193.
In order to accurately equalize into desired PR characteristics, a technique to automatically adaptively control equalizing coefficients (taps) of FIR filter is adopted. This technique is effective for various stresses such as tilting of a disc, defocus of laser beam, off-track of optical head. Many adaptive control algorithms are known, including Least-mean square (LMS) algorithm, Normalized LMS algorithm, Recursive Least Square (RLS) algorithm, projection algorithm, neural network algorithm.
Now, the adaptive waveform equalizer using LMS algorithm will be briefly described. In this algorithm, a temporary judgment value used for LMS is required in order to calculate adaptive equalizing coefficients. This LMS algorithm utilizes, a feedback operation for minimizing square errors between the “desired response” and the “response of transmission line.” This “desired response” is a PR equalization target value. The “response of transmission line” is a digital reproduction signal entered from FIR filter and equalized into PR frequency characteristics. In LMS algorithm, a signal that represents a difference between the temporarily judged value and the digital reproduction signal value after equalization, which is obtained in the block of adaptively controlling the coefficients of FIR filter is called an equalization error signal.
The block that adaptively controls coefficients of FIR filter updates the equalizing coefficients of FIR filter as required to minimize the square value of the equalization error signal. This is called as an adaptive equalization. A formula for setting LMS equalizing coefficients is shown in the following, formula (for example, S. Heikin: “Introduction to Adaptive Filters” Gendai Kogakusha):w(n(T+1))=w(nT)+A·e(nT)·x(nT)  Eq. (1)                (where, T=0, 1, 2, 3, . . . )        
w(nT) represents a present coefficient, w(n(T+1) is a coefficient to be updated, “A” is a tap gain, e(nT) is an equalization error, x(nT) is an input signal to FIR filter. “n” is a parameter for selecting update cycles of the coefficients. Based on Eq. (1) above, the equalizing coefficients of FIR filter is updated.
Now, asymmetry of optical disc 182 (FIG. 16) will be described. Asymmetry means absence of symmetry between pits and non-pits of the optical disc. In optical disc 182 (FIG. 16), information is recorded in the form of arrangement and length of microscopic emboss sections called pits. The pit has length of, for example, 3T, 5T when T denotes the reference length. Pits are arranged with spaces of 3T, 5T. The pit length is preferably exactly 3T, 5T. However, there are some deviations in pit length. This is because a master disc with deviations in pit length is manufactured as a result of, for example, slight deviation of power of recording light used for mastering an optical disc. When the recording power is not appropriate, each pit formed is slightly longer or shorter in the same amount from the standard value in front and behind in the length direction. That is, there is no symmetry between the pits and the nonpits, which is called as asymmetry. Hereinafter, in the present specification, relationships between the pits and the non-pits of the optical disc should be same as the relationships between recorded portions (marks) and unrecorded portions (spaces) of hard disk. Note that, for a read-only optical discs, terms “pit” and “non-pit” may be used and for recordable optical discs, the portion where the information is recorded (that is, portion intensely irradiated with laser) may be called as a “mark” and the region between marks a “space.” In the present specification, terms “pit” and “mark” are synonymous. Terms “non-pit” and “space” and further “non-mark” are synonymous. In addition, a signal when the optical disc with no symmetry between pits and non-pits (that is, asymmetry) is reproduced is called as an asymmetric signal, and a signal when not an asymmetric optical disc is reproduced is called as a symmetric signal.
FIGS. 18A through 18C show simple models of asymmetry. In FIGS. 18A through 18C, 3T marks, 3T spaces, 5T marks, and 5T spaces pit arrangements are shown. In these figures, the reference length is 1T and a detection window width is adopted. FIG. 18B is a standard pit arrangement, and both marks and spaces are symmetric. As against FIG. 18B, FIG. 18A shows marks, each of which has length uniformly shorter than that shown in FIG. 18B by length x. On the other hand, FIG. 18C shows marks, each of which has length uniformly longer than that shown in FIG. 18B by lengthy. In either case shown in FIGS. 18A and 18C, no symmetry is observed in both marks and spaces. Because this asymmetry is also caused by fluctuations of laser wavelength, in general, it is difficult to adjust and maintain symmetry between pits and non-pits at the time of recording.
Now, the description is made on the specific hardware configuration and procedure for binarizing the analog data signal (reproduction signal) read from the optical disc. FIG. 19 is a block diagram of the configuration of the PRML detector 210. PRML detector 210 carries out adaptive equalization and updates the equalizing coefficients of FIR filter from time to time. First of all, A/D converter 221 of PRML detector 210 converts the reproduction signal from analog to digital. Phase comparator 222 generates binary data with a certain threshold value used as a reference. Then, PR temporary judging section 223 receives the binary data. PR temporary judging section 223 temporarily judges the desired value of PR and outputs coefficient adaptive controller 224. The desired value of PR method can be determined based on the amplitude zero-cross information obtained at phase comparator 222 (for example, see Screen Image Information Media Society Technical Report (ITE Technical Report Vol. 24, No. 46, pp. 13-18 MMS2000-14 (July 2000)). Then, coefficient adaptive controller 224 updates the equalizing coefficient (tap) of FIR equalizer 225 using the adaptive algorithm described before. Viterbi decoder 226 converts the waveform equalized into the specified PR at FIR equalizer 225 into the binary data.
The clock used at A/D converter 21 is generated as a result of specified processing of phase comparator 22 which detects phase difference from the output of A/D converter 21 and by loop filter, DAC for converting digital signal to analog signal and voltage control transmitter VCO (all not illustrated).
FIG. 20 is a block diagram of the configuration of PRML detector 220. PRML detector 220 outputs the judgment value by PR (1, 1) equalization using the output of the FIR equalizer, and calculates the desired value of PR (a, b, b, a) equalization in the FIR equalizer using the judgment value, which is disclosed in, for example, Japanese Laid-open Publication No. 2000-123487. A/D converter 231 converts the reproduction signal from analog signal to digital signal. FIR equalizer 32 carries out the specified PR equalization for the digital signal. PR temporary judging section 233 temporarily judges the desired value of PR method using binarized data of output of FIR equalizer 32 and outputs to coefficient adaptive controller 234. Coefficient adaptive controller 234 uses the temporary judgment value and updates taps of FIR equalizer 232. PRML detector 220 can suppress the probability for occuring judgment errors to a low level by reducing the judgment threshold value.
Conventionally, since there used to be questions (1) and (2) as shown below, it was unable to obtain properly binarized reproduction signals. That is,
(1) When there is no symmetry between the pits and the non-pits in an optical disc (that is, asymmetry), the performance of PRML degrades. In other words, information reproducing apparatus 181 using the conventional PRML generates errors by asymmetrical reproduction signals.
Problem (1) above can be described as follows. FIG. 21A and FIG. 21B show histograms of output signals of A/D converter 185 in information reproducing apparatus 81 (FIG. 16). The abscissa is the reproduction signal level, while the frequency of signal level is taken as ordinate. This waveform example uses the 8-16 modulation used in the DVD (Digital Versatile Disc) specifications. That is, the reproduction waveform has 3T-14T (including sync code) mark length and space length.
The phase error is detected with the center of the reproduction signal level used as a reference (for example, central value 64 (40h) of expressible 0-128 when the number of effective bits of A/D converter is 7) and the clock frequency for sampling the reproduction signal and the phase are controlled. Under the control, histogram is separated to generally have five distributions. This is because when PRML has the PR coefficient such as a PR (a, b, b, a) ML, the number of signal levels (signal distributions) becomes five (let a, b denote positive coefficients). Waveform equalizer 86 controls the clock phase and facilitates PR equalization.
FIG. 21A shows a histogram of reproduction signal, which is not asymmetrical, while FIG. 21B shows a histogram of asymmetrical reproduction signal. FIG. 21C shows the histogram of output signal when waveform equalizer 86 PR-equalizes (in this case, PR (3, 4, 4, 3) equalizes) the non-asymmetrical reproduction signal (FIG. 21A). As clear from FIG. 21C, variance is minimum and each level is accurately separated.
On the other hand, FIG. 21D shows a histogram of an equalizer output when the asymmetric reproduction waveform is PR-equalized. As clear from the figure, PR-equalizing asymmetric reproduction waveform increases the variance. This is because PRML is originally intended for processing symmetric waveform and automatically equalizes the waveform in such a manner that each level has equal interval. In other words, when an asymmetric signal which has minimum variance with unequal intervals is supplied, the equalizer forcibly equalizes the signal to have equal intervals. As a result, the variance further increases.
FIG. 22 is a graph that shows the frequency characteristics of reproduction signal based on the histogram of asymmetric reproduction waveform of FIG. 21B. In addition, FIG. 22 also shows the frequency characteristics of the desired PR characteristics. In FIG. 21B, the left side from the center of the reproduction signal level is designated as mark and the right side as space. In the graph of FIG. 22, when the standardized frequency is taken as abscissa and the gain as ordinate, characteristics on the mark side and the characteristics on the space side differ each other due to the influence of asymmetry. As easily understood, in order to equalize the reproduction waveform to have desired PR characteristics, the equalizer must carry out equalization with characteristics that differ on the mark side and on the space side. However, the conventional waveform equalizer 6 cannot equalize the reproduction waveform to have desired PR characteristics highly accurately because the same equalization is carried out on the mark side and the space side. As a result, variance in the output signal of the maximum likelihood decoder 6 (FIG. 16) increases, resulting in degraded performance.
(2) When the adaptive equalization processing is automatically carried out, the results of the temporary judgment (temporary judgment value) utilized in LMS may cause errors. To specifically explain, first of all, in the system shown in FIGS. 19 and 20, when the reproduction signal contains comparatively less noise and provides good signal quality to a certain extent, satisfactory convergence characteristics can be obtained. However, when noises arising from various stresses as described above mix in the reproduction signal and the jitter increases, the bit error rate BER degrades and the probability for making an erroneous temporary judgment increases. Making an erroneous temporary judgment causes the equalization error signal to become abnormal and the LMS action itself becomes abnormal. Consequently, appropriate adaptive equalizing coefficients are unable to be calculated and correct PR equalization is prevented. This degrades the bit error rate of binary data after Viterbi decoding.